XSL FO: Property Expression and immediate value

Dear xsl-editors,

The XSL WD 2000-03-27 says:

    5.8 Expressions
    All property value specifications in attributes within an 
    XSL stylesheet can be expressions. 

How to distinguish expressions from immediate (non-expression) values?

The expressions and the immediate values are syntactically different,
but there are ambiguous points. 

In the expression syntax, the string values are quoted with single 
or double-quotes.  e.g., the "format" property may be specified:
format="'(1)'"  (the outer "" are XML attribute syntax's and inner '' 
are XSL expression syntax's).

We can also specify: format="(1)" as immediate value ("" are XML 
syntax's, not XSL expression's).  Here, the parentheses are part 
of string value (number-formatting: (1) (2) (3) ...), 
but if this is expression, the parentheses are syntax's
parentheses and the value is 1 (number-formatting: 1 2 3 ...)
It's very ambiguous.  How to resolve it?

Thanks,
MURAKAMI Shinyu